Knee injury and osteoarthritis outcome score (KOOS) – National record-based reference values

Background: Knee Injury and Osteoarthritis Outcome Score (KOOS) and the short form KOOS-12 are commonly used in clinical practice and research but there are no national record-based reference values to aid interpretation. The aim of this study was to establish national record-based reference values for the KOOS and its short form KOOS-12.

Patients and methods: A national record-based representative sample of 9996 adult citizens were derived from the Danish Civil Registration System. The selection of citizens was based on seven predefined age groups with an equal sex distribution across each age strata. The KOOS questionnaire was sent to all participants, together with two supplemental questions regarding previous knee problems and body mass index (BMI).

Results: A total of 2842 participants completed the KOOS, 1463 women (51.4%) and 1379 men (48.6%). The mean KOOS subscale scores were: pain 85.3 (95% confidence interval (CI): 84.6-85.9), symptoms 85.1 (95% CI: 84.5-85.8), activities of daily living (ADL) 86.7 (95% CI: 86.0-87.3), sport and recreation function 70.9 (95% CI: 69.8-72.0), quality of life (QOL) 74.9 (95% CI: 73.9-75.8).Age- and sex-specific reference values showed small differences in mean scores between the five KOOS subscales and all were below the threshold for clinically relevant improvement (10 points).Knee problems were associated with worse KOOS scores across all subscales. The difference in the mean subscale scores between the lowest (<24.9) and highest (>40) BMI groups ranged from 12.9 to 24.1. Comparable results were observed for KOOS-12.

Conclusion: KOOS and KOOS-12 reference values can, in most situations, be used without stratification for age and sex. Sport/recreation reference values stratified for age and BMI may be of importance.